In recent years, organic EL display panels having organic EL elements disposed on a substrate have become more commonly used as display devices. Since organic EL display panels use organic EL elements that emit light, the display panel is highly visible. Furthermore, each organic EL element is a complete solid state device and thus has excellent impact resistance.
Organic EL elements are current-driven light-emitting elements formed by laminating layers between an anode and a cathode electrode pair, such as an organic light-emitting layer that relies on the phenomenon of electroluminescence due to recombination of carriers. In an organic EL display panel, organic EL elements corresponding to the colors red (R), green (G), and blue (B) are formed as sub-pixels. One set of an R, a G, and a B sub-pixel constitutes a single pixel.
Among such organic EL display panels, a wet process (application process) for forming the organic light-emitting layer in each organic EL elements, such as an inkjet method, is well known (see, for example, Patent Literature 1). In the inkjet method, an inkjet head scans over apertures provided in a matrix in the layer of banks of the substrate (the apertures corresponding to regions for forming the organic light-emitting layers). A plurality of nozzles provided in the inkjet head eject drops of ink into each aperture. The ink includes organic material and solvent for forming the organic light-emitting layer. Normally, a drop is ejected multiple times into one aperture. In a piezo inkjet apparatus, the volume of the drops ejected from each nozzle is adjusted by changing the waveform of the driving voltage applied to the piezo element provided in each nozzle.
In an organic EL display panel, it is necessary for the luminance to be even between pixels. Since the luminance depends on the thickness of the organic light-emitting layer, it is necessary for the total volume of the drops ejected into each aperture to be even when the organic light-emitting layer is formed by the above method. Even if a drive signal of the same waveform is applied to each piezo element, however, variation occurs in the volume of the drops ejected from each nozzle, since the ejection characteristics of each nozzle differ. As a result, the total volume of the drops ejected differs for each aperture, causing variation between pixels in the luminance.
To address this problem, Patent Literature 1 discloses technology for detecting beforehand the volume of the drops ejected from each nozzle. Based on the result of detection of each nozzle, the waveform of the driving voltage applied to each piezo element is changed. This technology makes the volume of the drops ejected from each nozzle uniform, thereby making the total volume of the drops ejected into each of the apertures uniform.